The present invention relates to vehicle diagnostic systems, and more particularly to a method of testing for flow restrictions in an engine exhaust gas recirculation system.
Vehicle exhaust gas emissions are commonly reduced with an exhaust gas recirculation (EGR) valve controlled to allow engine exhaust gas to flow back into the intake air stream of the engine. The recirculated exhaust gas reduces peak temperatures within the combustion chamber, which in turn, reduces the formation of oxides of nitrogen (NOX). However, various exhaust gas passages in the EGR system may become restricted due to physical damage or formation of deposits (coking), allowing the peak combustion temperatures and NOX emissions to increase.
To satisfy diagnostic requirements, the engine controller is programmed to periodically run a diagnostic procedure that tests for flow restrictions and produces a fault indication if a debilitating restriction is detected. Typically, the procedure involves forcing the EGR valve to a predetermined opening when it would otherwise be closed, and measuring the resulting change in intake manifold absolute pressure. The EGR system is deemed to pass the test if the measured change in pressure exceeds a threshold based on the minimum expected change in intake manifold pressure for an EGR system that is regarded as functioning within acceptable limits, but is deemed to fail the test if the measured pressure change is below the threshold. Unfortunately, a large EGR valve opening is required to distinguish between a borderline passing restriction and a borderline failing restriction, and if the system is relatively unrestricted, the large opening results in a high EGR flow that can cause combustion instability and increase exhaust emissions. Using a smaller EGR valve opening to avoid these problems makes it difficult to reliably detect a debilitating restriction, and tends to increase the variability of the test results.
The present invention is directed to an improved test method for detecting EGR system restrictions which can reliably detect a debilitating restriction without significantly degrading combustion stability and exhaust emissions.
According to this invention, the EGR valve opening for diagnostic purposes (referred to herein as the test opening) is initialized at a relatively low value, which is progressively increased if the measured intake manifold pressure change fails to exceed a threshold based on the minimum expected change in intake manifold pressure for an EGR system which is regarded as functioning within acceptable limits. As soon as the measured pressure change exceeds the threshold, the EGR system is deemed to pass the restriction test, and the test method is terminated. If the EGR valve opening reaches a maximum value without the measured pressure exceeding the threshold, the EGR system is deemed to fail the restriction test, and a fault indication is generated. Preferably, the value at which the test opening is initialized is determined based on the results of a prior execution of the flow restriction test so as to minimize the duration of the test and its impact on engine operation.
With the test method of the present invention, the test opening of the EGR valve is adaptively determined based on the measured intake manifold pressure change, and is never larger than required to reliably detect a debilitating EGR restriction. Thus, the test opening remains relatively small if the EGR system is substantially unrestricted, but is capable of achieving a very large value if the EGR system is significantly restricted. This, in turn, allows reliable detection of a borderline failing EGR system with less test result variability, while preventing degradation of combustion stability and emissions in a substantially unrestricted EGR system.